1. Field of the Invention
The present invention relates to a method of controlling pressing force applied from an electrode to a workpiece (hereinafter referred to as electrode pressure application force) in an electric gun provided with the electrode at the tip end of a movable arm which is driven by an electric motor.
2. Description of Related Art
As a conventional method of controlling electrode pressure application force in an electric gun provided with the electrode at the tip end of a movable arm which is driven by an electric motor, an open loop control system by controlling the torque of the electric motor has been normally carried out.
For example, an electric gun is disclosed in JP-A 6-312273. In this application, a method of controlling electrode pressure application force is carried out in the following manner. One electrode is moved to a target position by a motor having a position detector, and a workpiece is clamped, pressed and held between one electrode and another electrode which are paired at a position where the electrodes and the workpiece contact. The current of the motor necessary for generating given pressure application force is measured at a plurality of spots, and transformation function and an inverse function thereof are prepared based on the group of currents of the motor. Then a motor current instruction value is calculated by an arbitrary setting pressure application force in a pressure application current instruction circuit, and an actual application force is calculated in response to a motor current signal which is fed back so as to convert from a welding motor current value to an actual pressure application force, wherein the amount of protrusion of one electrode is corrected when there is a difference between the arbitrary set application force and the calculated actual pressure application force.
However, in the prior art set forth above, the pressure application force is finally checked based on the motor current value which causes a problem that the accurate pressure application force is not checked owing to a delicate frictional resistance in mechanical parts between the motor to the electrode or change of conversion efficiency.